1. Field of the Invention
The present invention relates to a temperature-compensated crystal oscillator (TCXO) of a surface-mount type, and more particularly to a temperature-compensated crystal oscillator incorporating a direct temperature compensating circuit which comprises a thermally sensitive resistive element and a capacitor.
2. Description of the Related Art
Surface-mount temperature-compensated crystal oscillators are small in size and light in weight, and hence are widely used as frequency sources for communication devices in the mobile environment such as cellular phones. A surface-mount temperature-compensated crystal oscillator comprises a voltage-controlled crystal oscillator (VCXO) and a temperature compensating circuit for applying a control voltage depending on the ambient temperature to the voltage-controlled crystal oscillator.
Temperature compensating circuits that are available in the art include a direct temperature compensating circuit and an indirect temperature compensating circuit which differ from each other depending on how the control voltage is generated. The direct temperature compensating circuit comprises a thermally sensitive element, such as a thermistor whose resistance decreases as the ambient temperature rises, and a capacitor. The indirect temperature compensating circuit employs an active element for generating a temperature-compensated voltage depending on the ambient temperature.
FIG. 1 shows a circuit arrangement of a conventional temperature-compensated crystal oscillator, and FIGS. 2A and 2B are plan and side elevational views, respectively, of the conventional temperature-compensated crystal oscillator shown in FIG. 1.
The conventional temperature-compensated crystal oscillator comprises quartz crystal unit 1, oscillating circuit 2, frequency adjusting circuit 3, temperature compensating circuit 4, and AFC (Automatic Frequency Control) input circuit 5. These components of the temperature-compensated crystal oscillator are mounted on mounting substrate 7 which is provided with surface-mounting electrodes 6. Surface-mounting electrodes 6 serve to electrically connect the temperature-compensated crystal oscillator to a circuit pattern on a wiring board when the temperature-compensated crystal oscillator is surface-mounted on the wiring board.
As shown in FIG. 3, crystal unit 1 has casing 8 and crystal blank 9 housed in casing 8. Casing 8 is made of laminated ceramic and has a recess defined therein with a pair of connecting terminals 10 arranged on the bottom of the recess. Mounting terminals 11 for surface-mounting extend from an outer bottom surface to a side surface of casing 8, and are electrically connected to connecting terminals 10. Crystal blank 9 is of a substantially rectangular shape and has a pair of excitation electrodes (not shown) disposed respectively on opposite principal surfaces thereof. From the excitation electrodes, there extend respective extension electrodes toward respective opposite ends of one side of crystal blank 9. Crystal blank 9 is held in place in casing 8 by electrically conductive adhesive 12 by which the opposite ends of the one side of crystal blank 9 are bonded to connecting terminals 10 which are arranged on the bottom of the recess of casing 8. The recess in casing 8 is closed by metal cover 13, for example, which is joined to casing 8 by seam welding or the like, thus hermetically sealing crystal blank 9 in the recess in casing 8.
Referring back to FIG. 1, oscillating circuit 2 comprises an IC (Integrated Circuit) connected to one terminal of crystal unit 1. The IC comprises an integrated assembly of a split capacitor (not shown) cooperating with crystal unit 1 in making up a resonant circuit, an oscillating amplifier connected to the resonant circuit for feedback amplification, and a bias resistor. Bypass capacitor 14 is connected between power supply Vcc of oscillating circuit 2 and ground. Oscillating circuit 2 has an output terminal connected to coupling capacitor 15 through which oscillating output signal Vout is supplied to a next-stage circuit.
Frequency adjusting circuit 3 has a terminal connected to the other terminal of crystal unit 1, and comprises a parallel-connected circuit of two capacitors 16 for making coarse and fine adjustments, respectively. Each of two capacitors 16 comprises a chip capacitor.
Temperature compensating circuit 4 has a terminal connected to the other terminal of frequency adjusting circuit 3. As shown in FIG. 4, temperature compensating circuit 4 comprises a series-connected circuit of high-temperature compensating circuit 4a and low-temperature compensating circuit 4b which use the normal temperature as a reference temperature for their operation. Each of high-temperature compensating circuit 4a and low-temperature compensating circuit 4b comprises a parallel-connected circuit of thermistor 17 and capacitor 18, and compensate for temperatures based on a change in a capacitance (i.e., equivalent series capacitance) between its terminals which is caused when the resistance of thermistor 17 changes depending on the temperature. Each of high-temperature compensating circuit 4a and low-temperature compensating circuit 4b also has resistor 19 for adjusting the resistance of thermistor 17 at the normal temperature. In high-temperature compensating circuit 4a, resistor 19 is connected in series to the parallel-connected circuit of thermistor 17 and capacitor 18. In low-temperature compensating circuit 4b, resistor 19 is connected parallel to the parallel-connected circuit of thermistor 17 and capacitor 18.
AFC input circuit 5 has a first terminal connected to the other terminal of temperature compensating circuit 4 and a second terminal connected to ground. AFC input circuit 5 comprises a parallel-connected circuit of voltage-variable-capacitance diode 20, voltage divider resistor 21, and capacitor 22. AFC voltage Vf is applied to a third terminal of AFC input circuit 5 through high-frequency cutoff resistor 23. When AFC voltage Vf is applied, the capacitance of voltage-variable-capacitance diode 20 changes, changing and controlling the oscillating frequency of the voltage-controlled crystal oscillator. voltage divider resistor 21 controls AFC voltage Vf, and capacitor 22 controls the reference capacitance of voltage-variable-capacitance diode 20.
The temperature-compensated crystal oscillator with a direct temperature compensating circuit consumes less electric energy and has better phase noise characteristics of the oscillated output than a temperature-compensated crystal oscillator with an indirect temperature compensating circuit which generates a temperature compensating voltage using active elements, because the direct temperature compensating circuit is made up of passive elements only.
However, the temperature-compensated crystal oscillator with the direct temperature compensating circuit is basically constructed of discrete parts, and cannot easily be reduced in size, particularly, in planar profile dimensions, especially because temperature compensating circuit 4 including thermistors 17 and capacitors 18 cannot be integrated into an IC chip. The temperature-compensated crystal oscillator with the indirect temperature compensating circuit is more advantageous than the temperature-compensated crystal oscillator with the direct temperature compensating circuit with respect to a reduction in planar profile dimensions because the components except the crystal unit can easily be integrated into an IC chip. However, the temperature-compensated crystal oscillator with the indirect temperature compensating circuit consumes more electric energy and suffers more phase noise in the oscillated output.